I don't think isolationism is going to help the Fediverse thrive, or that there are no worthwhile users on Facebook platforms that could be persuaded to come this way if they get to see what it's like.
Having to deal with toxicity from that would suck, but it's not as if the Fediverse is this pure untainted land either. Worst comes to worst, instances can defederate them.
This is what I don't get about the anti-Meta paranoia. Their worst case scenario is... exactly the same as we have now. They can't make every single instance hand them over their whole userbases, that's not how federation works.
I get why they don't like Meta, but I have friends who use Facebook and Instagram, and maybe it would be nice if they could get a taste of the Fediverse experience. We might end up gaining more than Meta.